In recent times, brake systems of the brake-by-wire type were developed for motor vehicles, especially for passenger cars. In a brake system of this kind, the brake command of the driver is made apparent by applying a foot force to a brake pedal and the effect of the foot force on the pedal is detected by a sensor and converted into an electric signal. The signal is then transmitted to brake actuators of which one is assigned to each wheel of the motor vehicle and each brake actuator exercises, inter alia, a braking force on the brake lining with the aid of a braking-force device. The braking-force device is driven by an electric motor. The brake linings are pressed against the brake discs of the wheel brakes under the influence of the braking force whereby a braking force is generated at the wheel of the motor vehicle.
The braking force which is applied by the braking-force device must be controlled in dependence upon the brake command of the driver of the motor vehicle. A simpler possibility to do this comprises supplying a specific motor current to the electric motors of the brake actuators in dependence upon the brake command of the driver in consequence of which a specific braking force is applied by the braking-force device of the brake actuators.
This possibility is however problematic in that the brake actuators always exhibit an internal friction which leads to a hysteresis in the brake actuators. For this reason, it is not possible to clearly assign a braking force to a pregiven motor current; instead, for a pregiven motor current, the braking force always occurs at an undetermined point within a braking-force interval. Accordingly, only via a measurement of the braking force in the brake actuators can it be clearly and precisely determined which braking force is generated for a pregiven motor current by a braking-force device of a brake actuator. The measured value can be used for the purpose of adapting the actual braking force to the desired braking force corresponding to the brake command of the driver of the motor vehicle. For the above reasons, it is desirable that the braking force be measurable in the brake actuators as easily as possible.